Chng 2: Hin tng quang in

1. M u

Nghin

cu

cc

qu

trnh

tng

tc

bc x

- vt cht

Hiu

ng

quang

in, hiu

ng

Compton, s

sinh

cp: s

tn

x

hoc hp th

photon bi vt cht.

S

hy cp, qu

trnh

Bremsstrahlung: nhng

qu

trnh

sinh

photon

Cc

hin tng

ny

th

hin tnh ht

(photon) ca bc x

trong

tng

tc

ca

chng

vi vt cht, phn

bit vi tnh cht

sng

ca

bc x

khi

chng

la truyn.

Nhng

chng

sau, chng

ta

s

nghin

cu tng

qut

hn, l

lng

tnh

sng

ht ca vt cht, dn

n s

ra

i ca C

hc

lng

t

Chng 2: Hin tng quang in

2. Hiu

ng

quang

in

1886-1887, Heinrich Hertz ln

u tin thc hin

th

nghim chng

minh

s

tn ti ca sng in t

cng

nh

l

thuyt

in t

ca Maxwell v

s

lan

truyn

nh

sng.

Hertz khm

ph

rng, in

tch

gia 2 in cc s

suy

gim

nhanh

nu

chiu vo mt

trong

2 in cc

ny

nh

sng

t

ngoi.

Lenard

chng

minh

c hin tng

trn

l

do nh

sng

t

ngoi lm bt

electron ra

khi b

mt

cathode hiu

ng

quang

in

(photoelectric effect)

solar cell

Millikan (1914), Nobel prize 1923

Chng 2: Hin tng quang in

2. Hiu

ng

quang

in

Khi

kho

st

hot

ng

ca t

bo

quang

in, ngi ta nhn thy

Khi HT t

n gi tr

no

th

dng

quang

in c gi tr

bo

ha.

Khi HT c

gi

tr

m

l

V0

(gi l HT hm), dng

quang

in bt

u trit tiu

HT hm

khng

ph

thuc cng

sng; dng

bo t

l

vi cng

sng

Mi

kim

loi

(dng

lm

cathode) khc

nhau

c

mt tn s

gii hn

(cut-off frequency) n0

, di tn s

ny

hin tng

quang

in

khng

xy ra

0 maxeV K

Nhng

kt qu

trn

khng

th

gii

thch

bng

thuyt

sng

nh

c in:

Cng

sng

cng

mnh, bin

in trng

E cng

ln, lc tc dng

ln

electron cng

mnh, ng

nng

cng

ln. Tuy

nhin, V0

do Kmax

c lp vi cng

sng

Theo thuyt

sng

nh

sng, HTQ lun

xy ra vi mi tn s

nh

sng, ch

cn cng

sng

mnh

nh

bt

quang

electron, tc

khng

c

tn s

gii hn

Cn phi c mt khong

thi

gian

(time lag

thi gian tr), t

lc

chiu

nh

sng

cho

n

lc

c

quang

electron bt

ra, tuy

nhin

khng

c

khong

thi gian tr

no

c

quan

st

Chng 2: Hin tng quang in

3. L

thuyt lng

t

ca

Einstein v HTQ

L

thuyt lng

t

nng

lng

ca

Planck ch

gii hn

cho

cc

electron bc x

bn

trong

cc

thnh

hc. ng

cho

rng

nng

lng

in t, mt khi bc x, s

truyn

i mt

cch

lin

tc nh

sng

nc.

Nm

1905, Einstein m

rng

thuyt lng

t

Planck, ng

cho

rng

nng

lng

bc x

cng

b

lng

t

ha

thnh

gi

gi, sau

ny

c gi

l

cc

photon.

Einstein cng

cho

rng

cc

hin tng

nh

giao

thoa, nhiu x

nh

sng

l

do tp hp

mt s

rt ln

cc

photon. Hin tng

quan

st

c ca

nhiu x, giao

thoa

l

trung

bnh

ca

hnh

vi

ca

cc

photon khc

nhau.

Nm

1921Einstein c tng

thng

gii

Nobel cho

vic

tin

on

v

mt

l

thuyt ca cc nh

lut

quang

in

Chng 2: Hin tng quang in

4. Hiu

ng

Compton

Nm

1923, thm

mt bng

chng

v

tnh

cht ht bc x

t

th

nghim ca

Compton

Compton chiu chm tia X c bc

sng

l

ln tinh th

than ch

(graphite) v

o

nh

sng

tn

x

theo

cc

gc

khc

nhau. ng

nhn

c hai loi bc x

c

bc sng l

v

l >

l: hiu

ng

Compton

dch

bc

sng

Dl=l-l

thay

i

theo

gc

tn

x

Hiu

ng

Compton khng

th

gii thch bng

thuyt sng

in t

c in: sng

in t

tng

tc

vi

electrong

t

do, lm

cho

electron ny

dao

ng

cng

tn s

ci

n. Electron dao

ng

s

bc x

sng

in t

c

cng

tn s

dao

ng

ca

electron, do

cng

tn s

sng

in t n

Chng 2: Hin tng quang in

4. Hiu

ng

Compton

Compton cho

rng: chm

tia

X chiu ti

khng

phi mt sng vi tn s

n, m

l

tp

hp nhiu

photon,mi

photonmanh

nng

lng

hn.

Khi

photo tia

X tng

tc

vi electron, n s

truyn mt phn nng

lng

cho

electrong

ny, do

photon tn

x

c

nng

lng

nh

hn, bc sng ngn hn

Tnh

dch

bc

sng

Dl=l-

l

Chng

ta

tnh

ton

da trn thuyt tng

i

i vi

photon, v

khng

c

khi lng

ngh

nn

202 2

;1 /m cEv c

2 2 2 2 2

0( )E p c m c / / /p E c h c h

Chng 2: Hin tng quang in

4. Hiu

ng

Compton

Chng 2: Hin tng quang in

4. Hiu

ng

ComptonS

tn ti

photon tn

x

c

bc

sng

khng

i cn phi c mt gii thch

tha

ng

trn, chng

ta

cho rng

electron l

t

do, mc d tht s

n

lin

kt vi ht

nhn. Tuy

nhin

gi

thit trn l ng

nu

ng

nng

m

electron nhn

c sau tn x

l

ln hn rt nhiu so vi nng

lng

lin

kt ca

electron (vi ht

nhn)

Nu

electron lin

kt mnh

vi ht

nhn, hoc nng

lng

photon n

l

nh, khi

lectron

khng

b

gii

phng, khi

ta c th

xem

s

tn

x

l

tng

tc

gia

photon vi

ton

b

nghuyn

t.

Trong

trng

hp ny

dich

bc

sng

(Eq. (2.12)) l

khng

ng

k, do m0 c

thay

bng

M>>m0 (M l

khi lng

nguyn

t)

Qu

trnh

m

nhng

photon tn

x

khng

b

thay

i bc

sng

c

gi l tn x

Rayleigh

Nh

vy, tn

x

Rayleigh

chim

u th i vi

photon c

bc

sng

di. i vi

photon c

bc

sng

ngn

(tia

X, tia

Gamma), th

tn

x

Compton chim

u th, c bit l khi tm tn x

lm

bng

vt liu c

nguyn

t

s

nh

Chng 2: Hin tng quang in

5. Lng

tnh

sng

-

ht ca bc x in t

Chng 2: Hin tng quang in

6. Cch

to

photon v

tia

X

Chng 2: Hin tng quang in

7. Sinh

cp v hy cp

1. M u2. Hiu ng quang in2. Hiu ng quang in3. L thuyt lng t ca Einstein v HTQ4. Hiu ng Compton4. Hiu ng Compton4. Hiu ng Compton4. Hiu ng Compton5. Lng tnh sng - ht ca bc x in t6. Cch to photon v tia X7. Sinh cp v hy cp